1. Field of the Invention
The present invention relates to a constant current generation circuit, and more specifically to a constant current generation circuit composed of a current mirror circuit.
2. Description of Relates Art
A typical example of the constant current generation circuit of this type is disclosed in Japanese Patent Laid-open Publication No. Hei 1-321706. For example, the constant current generation circuit shown in FIG. 1 of this Japanese patent laid-open publication includes a constant current source 1 connected in series to an input current path of a MOS transistor current mirror circuit 2, and a bipolar transistor current mirror circuit 3 having an input current path connected in series through a load resistor R.sub.3 to an output current path of the MOS transistor current mirror circuit 2, an output current path of the bipolar transistor current mirror circuit 3 constituting an output of the constant current generation circuit.
The constant current source 1 includes a resistor R.sub.2 and three series-connected diodes D.sub.1, D.sub.2 and D.sub.3, which are connected in series between a ground and a low potential power supply line V.sub.EE. A constant voltage obtained across the three series-connected diodes D.sub.1, D.sub.2 and D.sub.3 is applied to a base of an NPN bipolar transistor Q.sub.4, so that a constant current I.sub.0 passes through the NPN bipolar transistor Q.sub.4. The value of this constant current I.sub.0 can be adjusted by a resistor R.sub.1 connected between an emitter of the NPN bipolar transistor Q.sub.4 and the low potential power supply line V.sub.EE.
In general, a current mirror circuit includes an input transistor and an output transistor, which have their gates or bases connected to each other. One end of a main current path of the input transistor and one end of a main current path of the output transistor are connected in common to the same power supply line. The gate or base and the other end of the main current path of the input transistor are connected to each other. With this arrangement, if a predetermined current is caused to pass through the input transistor, the output transistor is controlled to pass the predetermined current therethrough.
In FIG. 1 of Japanese Patent Laid-open Publication No. Hei 1-321706, current I.sub.0 passing through an input PMOS transistor T.sub.2 of the MOS transistor current mirror circuit 2 is transferred or copied to an output PMOS transistor T.sub.1 of the MOS transistor current mirror circuit 2, which supplies a constant current T.sub.1 to an input NPN bipolar transistor Q.sub.1 of the bipolar transistor current mirror circuit 3. In this case, the load resistor R.sub.3 functions to compensate for a drain voltage of the output PMOS transistor T.sub.1. In the bipolar transistor current mirror circuit 3, the current I'.sub.1 passing through the input NPN bipolar transistor Q.sub.1 is transferred or copied to an output NPN bipolar transistor Q.sub.2, so that a constant current I'.sub.2 is generated at a collector of the output NPN bipolar transistor Q.sub.2.
In the above mentioned constant current generation circuit, in the case that the two PMOS transistors T.sub.1 and T.sub.2 of the current mirror circuit 2 have an inclined current-voltage characteristics in a saturated region, if a drain voltage of the PMOS transistor T.sub.2 becomes different from a drain voltage of the PMOS transistor T.sub.1, the currents flowing through the respective PMOS transistors T.sub.1 and T.sub.2 becomes unequal.